Others have pointed out some of the benefits of having 2 retainers over 1 retainer with double capacity, but I think the actual reason it's this way are more along the lines of:

PS3 memory issues with that much storage. I don't know a lot about this, but it's something Yoshi is always throwing around when discussing increasing inventory and such.

Originally when 1.0 launched, you could only have 1 retainer and retainers were planned to be a paid service. They were planning to make it so you'd have to pay additional money per month to have more retainers. That was quickly abandoned when the game bombed, and then they gave everyone a second retainer. So that we can have 2 now is probably mainly just because that's the way it was in 1.0.

There wouldn't be enough tabs to make it not look awkward unless they made it really elongated. I personally love it because I can separate all my crafting materials in one and materia and to be made materia stuff on the other.

Exactly... I have one retainer with all my misc. animal bits (skins, bone, leaves, whatever my guy pulls off the carcass apparently) and another with my unconvertable/unsellable gear, pieces I'll use later on lower level classes, cool looking armor, and materia.

Market would be stronger if you were only allowed one retainer IMO. It would add in the ability to target a select group of buyers if the sellers were all grouped up in a location opposite of your retainer.